It is known that heat recovery steam generators (HRSGs) allow recovering heat from a hot exhaust stream of a gas turbine to produce steam that can be used either in a process (cogeneration) or in a steam cycle (combined cycle). The exhaust gases are made of a non-condensable part, mostly containing N2, O2, CO2 and Ar, and a condensable part made of water vapor. So far in prior art, the idea of fumes condensation has been discarded owing mainly to corrosion concerns.
It follows that HRSGs as known in the prior art face two types of limitations in their thermal performance. In the case of HRSGs providing a stream of pressurized hot water for cogeneration applications, as exemplified in FIG. 1 and FIG. 2, a trade-off exists between the amount of thermal power and the amount of electricity generated by the steam turbine (which is directly related to the steam production of the HRSG). This trade-off is sketched in FIG. 3 where the point “Prior art 1” relates to the HRSG arrangement in FIG. 1 and the point “Prior art 2” relates to the HRSG arrangement in FIG. 2. On the other hand, the presence of sulfur compounds raises dramatically the acid dewpoint of the exhaust gases. As a result, a large recirculation loop has to be used to warm the feedwater above this acid dewpoint. This leads to a rather large approach, i.e. the difference between the temperature at the outlet of the preheater and saturation temperature of the low pressure steam, as shown in FIG. 4.
Some prior art documents rely on a secondary/by-pass stack where additional heat recovery is performed. In WO 2015/039840 A2, extra recovered heat is used to preheat the condensate of the steam cycle. In WO 2010/136795 A2, extra recovered heat is used to preheat the combustion air of the boiler. Both documents are however specifically oriented to boilers fed by air and fossil fuel (e.g. coal, oil), and are not concerned with heat recovery steam generators included in a combined cycle. Both patents lead to improvement of the efficiency of the cycle by reducing the amount of fuel needed to produce a given amount of steam at prescribed conditions of temperature and pressure. Document CN101922821 A discloses a method for simultaneously recovering water and latent heat in high-humidity flue gas, and an absorption heat pump device, relating to the technique of energy-saving equipment. This prior art solution is however expected to provide acidic corrosion deposits.